Inland waterways offer a cost-effective, energy-efficient and relatively safer mode of freight transportation, and autonomous navigation presents an attractive opportunity to fully revitalise their utilisation. To enable a safe, efficient and reliable inland waterway ecosystem, a
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Inland waterways offer a cost-effective, energy-efficient and relatively safer mode of freight transportation, and autonomous navigation presents an attractive opportunity to fully revitalise their utilisation. To enable a safe, efficient and reliable inland waterway ecosystem, autonomous inland vessels must account for uncertainties arising from environmental disturbances and modelling errors. In comparison to open-sea navigation, inland navigation involves confined waterways, frequent interaction with infrastructure and other vessels, and operational constraints that require both high situational awareness and constraint satisfaction. Furthermore, abnormal operating conditions resulting from critical sensor faults and failures must be tolerated through graceful performance degradation or, in the worst case, a fallback operation. In light of these operational requirements, this thesis investigates how autonomous vessels, especially (but not limited to) inland waterway vessels, can maintain safe, high-performance motion control while monitoring sensor faults and hazardous situations that affect their navigation.
More specifically, this thesis contributes an integrated framework that includes (a) a robust system identification methodology to obtain vessel maneuvering models for state estimation and prediction, (b) a Nonlinear Model Predictive Control (NMPC)-based control system that computes the vessel's control actions while satisfying the physical and operational constraints of inland waterways, (c) a multiple sensor Fault Detection and Isolation (FDI) scheme that monitors consistency in measurements by employing analytical redundancy relations and (d) a risk mitigation method that provides a fallback control action under complex failures.
Robust system identification for marine surface vessels
Maneuvering models play a central role in model-based control and monitoring system design by providing accurate estimates of the vessel's states and their future predictions. Identifying the parameters of a full-scale vessel from experimental data is particularly challenging due to significant modelling and measurement uncertainties. The first contribution of this thesis is a set-membership method for identifying key parameters of a nonlinear 3-Degrees of Freedom (3-DOF) vessel model that supports robust prediction and control design through a bounded error characterisation of the uncertainties. The identification process involves computing two sets: a Data-driven Parameter Set (DDPS) and a Feasible Parameter Set (FPS), using the system dynamics, uncertainty bounds and input-output measurements. Then, by solving quadratic programs over the FPS, parameter estimates and their uncertainty bounds are obtained. Validation results from full-scale trials demonstrate improved prediction accuracy and reduced computational time. In addition, through sensitivity analysis, the parameters most crucial for identification performance are identified.
Path-following control of inland waterway vessels in confined waterways
Inland waterways are characterised by tight operational and environmental constraints, leading to explicit control design specifications. The model predictive control methodology is adopted, as it naturally integrates multi-variable dynamics, actuators, state, environmental constraints and objectives to optimise performance and control effort. An NMPC path-following control scheme is proposed for Inland Waterway Vessels (IWVs), with the prediction model tailored to the hydrodynamic phenomena in confined waterways, including bank and shallow-water effects. Many challenging scenarios are considered for validating the control scheme through simulations, such as turning at a steep river confluence, sailing a curved river and avoiding a static obstacle. The impact of reduced ship-bank distances, propulsion speeds and river cross-section shapes further provides insights into control performance and design choices. In addition, key performance metrics are proposed to evaluate the controller's performance and quantify path-following accuracy, robustness and safety.
Multiple sensor fault diagnosis of autonomous surface vessels
Autonomous vessels rely on multiple heterogeneous sensors for navigation, motion control and situational awareness. Sensor faults may propagate through measurements to interconnected systems on board, thereby impacting downstream decisions. This thesis proposes a multiple-sensor FDI scheme that exploits Analytical Redundancy Relations (ARRs) derived from the vessel's dynamical model and adaptive thresholds to diagnose sensor faults.
The design methodology adopted in the proposed scheme includes (a) the generation of fault detection residuals having structural sensitivity to one or more sensor faults and (b) the computation of adaptive thresholds used for residual bounding with robustness against environmental and modelling uncertainties. As a result, false alarms can be avoided in the fault detection process. In addition, a combinatorial fault decision logic is designed, enabling the scheme to not only detect fault occurrence but also to determine the compromised sensors. Combined, the structurally sensitive residuals and the decision logic facilitate the isolation of multiple sensor faults. The proposed fault diagnosis scheme is suitable for continuous monitoring of faults during vessel operation, while easily accommodating variations in the vessel's actuator or sensor configurations. Furthermore, by identifying weak fault sensitivity by evaluating residuals with respect to fault magnitudes, improved fault isolation decisions are obtained.
Collision and grounding risk mitigation of inland waterway vessels
Finally, the risk mitigation of autonomous vessels is explored by considering the underlying sub-problems of risk modelling and control. For risk modelling, a Bayesian Belief Network (BBN) is built from hazard analysis results, providing transition probabilities for sequential decision-making. Thereafter, a Partially Observable Markov Decision Process (POMDP) model is designed to represent the vessel's states and provide a suitable higher-level control strategy that ensures the vessel's safety by preventing hazardous situations, such as grounding and collisions. The method is verified through an inland waterway navigation case study, which demonstrates SCS selection reliably during a complex failure scenario.